In the field of magnetic recording using heads and media, there is demand for further improvement in the performance of magnetic recording media and magnetic heads in association with the high recording density of a magnetic disk device.
The magnetic recording medium is a discontinuous medium where magnetic grains aggregate, and where each magnetic grain has a single magnetic domain structure. In the magnetic recording medium, one recording bit is configured of a plurality of magnetic grains. Consequently, in order to enhance the recording density, unevenness at the boundary of adjacent recording bits must be diminished by reducing the size of the magnetic grains. However, if size of the magnetic grains is reduced, the problem where the thermal stability of magnetization of the magnetic grains is reduced in association with the reduction of the volume of the magnetic grains.
As a countermeasure against this problem, an increase of uniaxial magnetic anisotropy energy Ku in the magnetic grains can be considered, but the increase of Ku causes an increase of a magnetic anisotropy field (coercive force) of the magnetic recording medium. In the meantime, the upper limit of the recording field intensity by the magnetic head is primarily determined according to saturation magnetic flux density of a soft magnetic material that configures a magnetic core within the head. Consequently, if the magnetic anisotropy field of the magnetic recording medium exceeds the tolerance value determined from the upper limit of the recording field intensity, it is impossible to record onto a magnetic recording medium.
At present, one method for solving the problem of thermal stability is energy assisted recording in which a magnetic recording medium formed with a magnetic material with large Ku is used. In this method, it is proposed to provide supplemental energy to the medium at the time of recording to effectively decrease the recording field intensity. The recording method which uses a microwave magnetic field as the supplemental energy source is referred to as microwave assisted magnetic recording (MAMR), and research and development are in progress for practical uses.
In the microwave assisted magnetic recording, the application of the microwave magnetic field in the medium in-plane direction of a frequency according to an effective magnetic field (Heff) relating to magnetization of a recording layer in the magnetic recording medium induces precession movement of the magnetization of the recording layer, and recording capability is assisted by a magnetic head.
As one example of the magnetic head using a microwave assisted magnetic recording method, a magnetic head is proposed that includes a main magnetic pole that generates a recording magnetic field for applying to the magnetic recording medium, a trailing shield, and a spin torque oscillator (STO) that is provided between them (write gap) and has a multilayered structure of a magnetic thin film. In the magnetic head, a microwave magnetic field in the in-plane direction is generated due to the self-excited oscillation of an STO, precession movement of the magnetization of the recording layer is induced by applying the microwave magnetic field to the magnetic recording medium, and magnetization reversal in the perpendicular direction in the recording layer is assisted.
In general, the STO has a multilayered structure where an under layer having a seed layer and a buffer layer, a spin injection layer (SIL), a spacer layer, a field generation layer (FGL) and a cap layer are laminated in respective order from the main magnetic pole side or the trailing shield side. In the STO, if a spin polarized current generated in the magnetic layer of the SIL is injected into the FGL, the magnetization of the FGL is oscillated using spin torque and microwave magnetic field is generated. The application of the microwave magnetic field to the magnetic recording medium redundantly with the recording field from the main magnetic pole enables reduction of the coercive force of the magnetic recording medium, and it becomes possible to record onto the magnetic recording medium.
The SIL requires high spin polarization and high perpendicular magnetic anisotropy, and as a magnetic material that can satisfy the requirements, for example, a [CoFe/Ni]n multilayered film is effective. Here, n indicates the number of repeated laminations of the lamination structure indicated within the bracket. Conventionally, STO using a [CoFe/Ni]n multilayered film as the SIL is proposed (U.S. Pat. No. 8,920,847). However, the perpendicular magnetic anisotropy of the SIL made from the [CoFe/Ni]n multilayered film is easily influenced by the crystal orientation or crystallinity of the multilayered film, with the problem that the perpendicular magnetic anisotropy of the SIL varies according to not only film formation conditions for the multilayered film, but also a constituent material of the under layer where the multilayered film is formed.
Further, in the microwave assisted magnetic head, the STO is provided in the write gap between the main magnetic pole and the trailing shield, and in order to accomplish high recording density, narrowing of the write gap, i.e., thinning of the STO provided in the write gap, is required.
In U.S. Pat. No. 8,920,847, a multilayer with tantalum (Ta) and copper (Cu) is disclosed as an under layer; however, if the under layer becomes thinner, it becomes difficult to form SIL having high perpendicular magnetic anisotropy, and the problem of narrowing of the write gap cannot be solved.
Consequently, the proposal of a new material in which a magnetic layer, such as SIL, having a desired magnetic characteristic(s), such as high perpendicular magnetic anisotropy can be laminated and an under layer can be configured to be thinner, is in demand.